Just read an interesting article about the challenges of getting a camping site in State Parks during the summer: http://freakonomics.blogs.nytimes.com/2010/01/25/our-daily-bleg-help-honest-michiganders-get-their-camping-fix/

Yes it looks like in Missouri reservations can be made up to six months in advance or as close as two days prior to your arrival. Reservations cannot be accepted later than two days in advance of your arrival. State Park reservation link here (http://www.mostateparks.com/campres.htm)